1. Field of the Invention
This invention relates to a transponder and, more particularly, to an improved radar transponder for transmitting frequency coherent coded pulses in response to an interrogating pulse.
2. Prior Art
Radar beacon systems are extensively used in navigation for indicating the ranges and positions of hazards along navigation routes. Often, a radar beacon is designed to function as an active reflector and includes a transponder that receives an interrogating pulse, and transmits a reply pulse to an interrogating radar which may be mounted on-board a ship navigating along its route. Present day marine radar beacons utilize a transponder adapted to provide a variable frequency oscillator to slowly sweep the RF frequency band (e.g., 9300 to 9500 megahertz) allocated for navigational purposes. In order to cater to the different frequencies of the various types of the short wave length radars on board the ships within this frequency band, typically the transponder is designed to transmit a coded RF signal when --and only when-- the beacon's swept oscillator matches the interrogating radar's transmission frequency. It has been found that with the use of such radar beacon systems, the probability of returning a reply to the interrogating radar on each of its antenna revolutions is rather low, and this has been found to be objectionable especially for ships in approaches to the archipelago. Further, these radar beacon systems suffer in a multi-user environment where, for example, many ships with radars operating at different frequencies within the frequency band are simultaneously interrogating the beacon.